This invention relates to high-temperature machine components. More particularly, this invention relates to coating systems for protecting machine components from exposure to high-temperature environments. This invention also relates to methods for protecting articles.
The components of gas turbine assemblies and other industrial equipment can be exposed to gas temperatures in excess of 1350° C. Accordingly, such components are designed to reliably perform their functions within this aggressive service environment, often employing a combination of strategies to prolong nominal service time at temperature. For instance, high temperature materials such as superalloys are often employed in turbine assembly components exposed to the flow of hot gas. Additionally, thermal barrier coating systems, generally comprising an oxidation-resistant metallic “bondcoat” disposed on the component surface and a heat-resistant ceramic “topcoat” disposed over the bondcoat, are often used to maintain somewhat reduced temperatures in the underlying component material. Furthermore, such components often employ cooling systems that may include complex arrangements of internal cooling channels that receive air or other cooling fluid and circulate the fluid throughout the component to maintain its temperature at an acceptable level. The internal cooling channels may connect to the outer surface of a component through multiple orifices, often referred to as “cooling holes,” dispersed over the surface. In some cases these cooling holes may allow the exiting fluid to form a film over at least a portion of the surface and thereby provide cooling or insulation to the surface (“film cooling”). In other cases, the flow of air or other fluid out of cooling holes is used to provide cooling by convection or impingement cooling.
Thermal barrier coatings are deposited using a variety of techniques, including physical vapor deposition (PVD) and air plasma spraying (APS). APS is much more economical than PVD due to its much higher nominal deposition rates and less expensive capital equipment requirements. However, it is known that conventional APS techniques frequently produce clogging of cooling holes, and thus in many applications, particularly those with fine cooling holes on the order of 1 mm and smaller, the more expensive PVD techniques are required to adhere to critical cooling hole flow specifications. Even components with larger cooling holes that are coated via APS often require post-coating processing to clear holes clogged by the spray process.
There is thus a need for economical coating processes that allow economical coating application using spray techniques while avoiding or minimizing the problem of cooling hole clogging, and for components having durable, reliable economical coatings with minimized restrictions in cooling fluid flow from their cooling holes.